Originally Posted by pizzaguy

Just keep in mind, my coil went open without warning. DJ told me he's seeing a LOT of this recently. As to burned/charred relay contacts, DJ says that is often the AIr Pump relay, as it is passing considerable current and, since it goes to a motor, the inductive kick when the relay opens causes a small arc. The other relays are not subject to that as they are supplying power to non-inductive loads.

And this is where MB was smart, notice how the box has that fan in it that draws filtered air from behind the cabin filter. This means the box is slightly pressurized. This has two effects:
1) The box is slightly above atmosphere, so dust and contamination from the underhood area does not get into the box.
2) The box stays pretty cool inside even when it's HOT out.

That last item was really proven to me when I took the car two weeks ago and had to get into the box to get the car to re-start at the bank. I did not put the cover on, drove 4-5 minutes back to the house, and right away yanked the RCM out - it was pretty warm just from those few minutes of unprotected operation. I"ve pulled a lot of RCMs out of cars after being shut off, they's always been very cool to the touch, thanks to the design.

Not ALL of MBs complicated techniques are fails - the fan in the box is really effective.